According to a new estimate of the rate of star formation, the Milky Way is ejecting far more stars than previously thought.
Gamma rays from aluminum-26, a radioactive isotope that originates mostly from massive stars, show that the Milky Way turns four to eight solar masses of interstellar gas and dust into new stars each year, researchers reported in a paper submitted to arXiv.org in January . 24. This range is two to four times the traditional estimate and corresponds to an annual birth rate of about 10 to 20 stars in our galaxy, since most stars are less massive than the Sun.
At this rate, every million years—the blink of an eye in astronomical terms—our galaxy produces 10 to 20 million new stars. That’s enough to fill about 10,000 star clusters, such as the beautiful Pleiades cluster in the constellation Taurus. In contrast, many galaxies, including most of those orbiting the Milky Way, do not form new stars at all.
“Understanding the rate of star formation is very important for the evolution of galaxies,” says Thomas Siegert, an astrophysicist at the University of Würzburg in Germany. The more stars a galaxy creates, the faster it becomes enriched with oxygen, iron, and other star-forming elements. These elements then change the star-forming gas clouds and can change the relative abundance of large and small stars that form the gas clouds.
Siegert and his colleagues studied the observed intensity and spatial distribution of aluminum-26 radiation in our galaxy. A massive star creates this isotope during life and death. During its life, the star blows aluminum into space with strong winds. If a star explodes as it dies, a supernova occurs. An isotope with a half-life of 700,000 years decays and emits gamma rays.
Like X-rays, unlike visible light, gamma rays penetrate the dust that covers the youngest stars. “We’re looking at the entire galaxy,” says Siegert. “We don’t x-ray it; here we are conducting gamma radiation.”
The more stars our galaxy produces, the more gamma rays appear. The researchers believe that the star formation rate of four to eight solar masses per year best matches the observations. This is much higher than the standard estimate for the Milky Way, which is about two solar masses per year.
The revised rate is very realistic, says Pavel Krupa, an astronomer at the University of Bonn in Germany who was not involved in the work. “I was very impressed with the detailed modeling of how they explain the star formation process,” he says. “This is a very beautiful work. I can see some ways to improve it, but it’s a really big step in absolutely the right direction.”
Siegert cautions that it’s hard to tell how far the gamma rays traveled before reaching us. In particular, if some of the observed emission originates nearby—just a few hundred light-years away—then there is less aluminum-26 in the galaxy than the researchers estimated, meaning the star formation rate is on the low side of the new estimate. However, he says it is unlikely to be as low as the standard two solar masses per year.
Either way, the Milky Way is the most active star producer in a collection of more than 100 nearby galaxies called the Local Group. The largest galaxy in the Local Group, Andromeda, turns only a fraction of the Sun’s mass of gas and dust into new stars per year. The Milky Way is the second largest of the Local Group galaxies, but its high rate of star formation means we’re definitely pushing harder.
